Oil and natural gas are common fossil-based resources used for the production of transportation fuels, heat and power, materials, chemicals, adhesives, pharmaceuticals, polymers, fibers and other products. Since the first oil well drilled in 1859 and the introduction of the internal combustion engine, the United States has been a major producer and consumer of fossil resources (Drake Well Museum, 2012).
In 2010, the US produced over 2 billion barrels of oil and 26.8 trillion cubic feet of natural gas worth over $180 and $110 billion, respectively. A significant amount of this production can be attributed to advances in horizontal drilling and hydraulic fracturing. Previously unrecoverable deposits have been freed up ensuring access to decades of domestic natural gas and oil. In fact, without hydraulic fracturing, 17% of oil and 45% of natural gas production would be lost within five years (IHS Global Insight, 2009).
Oil and natural gas deposits are located all across the United States and the World. It is estimated that the total amount of technically recoverable natural gas resources worldwide is 22,600 trillion cubic feet of which shale gas is 6,622 trillion cubic feet or nearly 30% (U.S. Department of Energy and Energy Information Administration, 2011). Wells are drilled hundreds of meters deep in order to gain access to the resources. Once drilled, new wells or old unproductive wells are hydraulically fractured to stimulate production.
Drilling fluids or muds are used during the initial well bore to cool the bit, lubricate the drill string, suspend and transport cuttings, control hydrostatic pressure and maintain stability. Drilling fluids are typically water- or oil-based but can be pneumatic. Water or oil is the main ingredient in liquid drilling fluids. Barite, clay, polymers, thinners, surfactants, inorganic chemicals, bridging materials, lost circulation materials and specialized chemicals are also added to engineer drilling fluid properties. Drilling fluids make up between 5-15% of drilling costs (Ben Bloys, 1994).
Hydraulic fracturing was developed in the 1940's to increase productivity of oil and gas wells. Hydraulic fracturing creates and maintains cracks within oil and gas formations providing a clear path for oil and gas to flow. Fracturing can be performed in vertical and horizontal wells. During a fracturing operation, perforations are made through cement casing into the oil and gas formation using explosive charges. Fracturing fluids are injected into the well at high pressures to create new cracks while further expanding and elongating the cracks formed by the explosives (American Petroleum Institute, 2010).
Fracturing fluids are composed primarily of water (87-94%) and proppant such as sand (4-9%). Sand mixed with the fracturing fluids is used to prop open formation cracks and maintain a clear path for oil and natural gas. The remaining fracturing fluid (0.5-3%) is composed of chemicals that aid the fracturing process. Chemical additives are mixed into the drilling fluid depending on the well and formation properties. Chemicals are used to dissolve minerals, reduce friction, prevent scaling, maintain fluid properties (viscosity, pH, etc.), eliminate bacteria (biocide), suspend the sand, prevent precipitation of metal oxides, prevent corrosion, stabilize fluid, formation and wellbore, thicken fluid (gelling agent) and breakdown the gel (breaker) (American Petroleum Institute, 2010).
Hydraulic fracturing fluid is made in a step-wise procedure and carefully engineered to accomplish the fracking process. In its most basic form, a gelling agent such as gaur gum is first added to water and hydrated. Next a breaker (oxidant or enzyme) is added which will break the gel bonds after being pumped into the well. A crosslinking agent such as borate is then added to the solution which immediately forms a viscous, gelled solution. The purpose of the gel is to suspend the proppant while being pumped into the well where it is wedged into formation fractures propping them apart.
Eventually the fracturing fluid must be removed from the well leaving the proppant in the fractures to maintain open channels for oil or gas to flow through. In order to pump the fracturing fluid out of the well and leave the proppant behind the viscous gel must be broken down to a viscosity less than 100 cP. Since the fracturing fluid is pumped into the well in stages, precise amounts of breaker are mixed with the fracturing fluid to break the entire gel solution simultaneously. Once the entire gel is broken the fracturing fluid is pumped back to the surface where it is stored in retention ponds or hauled away from the well for treatment and disposal.
One of the challenges associated with drilling and hydraulic fracturing is in using oil-based fluids. Oil-based fluids are subject to environmental scrutiny and are costly since they may include substantial quantities of refined petrochemicals or fuels, such as diesel fuel.
Another challenge associated with drilling and hydraulic fracturing is reducing the amount of water used in the process. Depending on how deep the well is, millions of gallons of water may be used during both drilling and hydraulic fracturing (Ground Water Protection Council and ALL Consulting, 2009). Recovered fluid is stored in open retention ponds where it is left to settle and evaporate or trucked out to be treated. In some cases, fluid is lost underground.
Another challenge associated with drilling and hydraulic fracturing arises from the retention ponds themselves. Retention ponds contain the recovered fluid, chemicals and cuttings from the well. Retention ponds may be a source of ground water contamination if the containment area gives way or the liner is pierced. Retention ponds also pose risks to wildlife if exposed to chemicals. Furthermore, if wells are planned in environmentally sensitive areas, retention ponds may not be permitted. In this case cuttings and recovered fluids and chemicals must be transported from site for treatment.
A criticism with drilling and hydraulic fracturing relates to the types of chemicals used during the process and the hazards they may pose. Though usually less than a couple percent of the entire fluid, common chemical additives include hydrochloric acid, formic acid, citric acid, boric acid, acetic acid, lauryl sulfate, polyacrylamide, ethylene glycol, borate salts, potassium carbonate, potassium chloride, glutaraldehyde, guar gum, isopropanol, petroleum distillates, sodium chloride, methanol and 2-butoxyethanol.
Another criticism associated with drilling and hydraulic fracturing is that groundwater contamination may occur in underground aquifers. During drilling and hydraulic fracturing, cement and steel casing is placed in the wellbore as a boundary between aquifers and the production material. Nonetheless, many have complained about chemicals found in drinking water and blame drilling and hydraulic fracturing. The Environmental Protection Agency and other agencies are investigating if and how drilling and hydraulic fracturing are linked with ground water contamination (Environmental Protection Agency, 2009).
Another criticism associated with hydraulic fracturing is ground water contaminated with natural gas. Landowners cite detectible amounts of thermogenic methane in drinking water that contribute to poor health of livestock and humans (Stephan G. Osborn, 2011).
In U.S. Pat. No. 5,067,566 (Nov. 26, 1991) a subterranean formation fracturing method is disclosed where a hydratable polymer, crosslinking agent and breaker are combined to form a fracturing fluid. A pH regulating substance is added to the fluid which slowly hydrolyzes forming an acid so that the breaker is activated and can control the breaking point of the polymer gel. The patent notes that one of the challenges with breakers is that they are difficult to control such that the entire gel is broken simultaneously. Oxidant breakers are ineffective at temperatures below 55° C. without added coreactants and, although enzyme breakers can be used at lower temperatures, they are sensitive to pH. The invention seeks to improve upon these breakers, however it requires an additional pH regulating substance for enzymatic breakers to be effective.
In U.S. Pat. No. 5,624,886 (Apr. 29, 1997) a subterranean formation fracturing method is disclosed where a hydratable polymer, crosslinking agent and breaker are combined to form a fracturing fluid. The breaker is made of an insoluble oxidant and formed as a pellet. However, the insoluble breaker is not evenly mixed with the fracturing fluid which may prevent even breaking throughout the fluid. Furthermore, a solid, insoluble, pelletized breaker is costly to manufacture and difficult to incorporate into liquid fracturing fluids.
In U.S. Pat. No. 7,231,976 (Jun. 19, 2007) a method of treating a well with a biodegradable fluid consisting of a lactic acid ester and a fatty acid ester is disclosed. The biodegradable fluid may be emulsified with alcohol and water using emulsifiers and used to remove pipe dope, hydrocarbons and drilling muds. Furthermore, the biodegradable fluid may act as a breaker catalyst to decrease the viscosity of fracturing fluids or replace synthetic and oil based drilling muds. However, forming an emulsion adds increased complexity and cost to a drilling and fracturing operation.
In U.S. Pat. No. 5,678,632 (Oct. 21, 1997) a method of acidizing an underground reservoir by injecting a substrate and enzyme which converts the substrate into an organic acid is disclosed. However, it is noted that the enzyme may be inactive under certain temperatures, pressures and environments and fail to treat the reservoir.
In U.S. Pat. No. 5,639,715 (Jun. 17, 1997) an environmentally non-toxic drilling fluid is disclosed. The drilling fluid is made in part from a surfactant that imparts anti-bit balling, lubricity, salt tolerance and non-toxic properties. However, the non-toxic surfactant is only an additive to the drilling fluid which may include other toxic compounds.
What are needed in the art are methods or products that minimize environmental impact, scrutiny, water-use and costs of drilling, treating and hydraulic fracturing for oil and gas. Furthermore, products that are environmentally benign, have reduced toxicity and do not contaminate drinking water are needed. A preferred fast pyrolysis process that converts biomass into renewable bio-oil fractions and carbon-rich biochar will reduce water use, hazardous chemicals and fluid cost while improving the environmental sustainability of drilling, treating and hydraulic fracturing for oil and gas.